Well, as of the latest manga chapters, Yamato has indeed become a part of the Straw Hats. This decision brings new dynamics and adventures to the crew.
One great rival could be a character with a powerful Devil Fruit ability. For example, a character who has a Logia - type Devil Fruit that allows them to turn into a natural element like lightning. This would make them very difficult to defeat in combat as they could dodge attacks easily. They could also use their element - based powers to attack the Straw Hat Pirates in unexpected ways, such as sending bolts of lightning towards their ship.
It would be a wild combination. Vegeta's serious and prideful nature would contrast with Luffy's carefree attitude. But his combat skills would be a great asset. He could probably take on the toughest enemies easily. He might not get along well with Usopp at first because of Usopp's cowardly tendencies, but over time, he could teach Usopp to be braver. And his presence would make the Straw Hat crew even more feared among their enemies.
First, understand the Straw Hat Pirates' personalities well. For example, Luffy's carefree and determined nature. Then, create an OC (Original Character) that can interact meaningfully with them. Maybe the OC has a special skill or a unique backstory that ties into the pirates' adventures. Think about how the OC can join their crew or be an ally in their journey.
One popular trope is the 'lost relative' trope. For example, the OC could be a long - lost relative of one of the Straw Hat Pirates. Another is the 'powerful newcomer' trope, where the OC has some extraordinary power that helps the crew in a difficult situation. There's also the 'love interest' trope, although this can be a bit overdone. The OC might fall in love with one of the pirates or vice versa.
The order in which the main characters of One Piece joined the Grass Hat Pirates was as follows:
1 Monkey D Luffy
2 Nico Robin
3 Franky
4 Choba (Chopper)
5 Brook
6 Sauron (Sengoku)
7 Nami
8 Usopp
9 Sanji
10 Nico Robin's father
The above are the original members of the Hat Pirates. As the plot developed, Luffy, Nami, Usopp, Sanji, Sauron, Choba, and Franky joined one after another. Nicole Robin also joined the Hat Pirates at some point later.
The Straw Hat crew would probably be very surprised at first. Nami might worry about how Hancock's presence would affect their finances, since Hancock is used to a more luxurious lifestyle. Usopp would be in awe of Hancock's beauty and powers. Chopper would be excited to have Hancock around, as she could potentially teach him about different kinds of medicine from Amazon Lily. Zoro might be a bit skeptical at first but would come to accept it as long as Luffy was happy.
One idea could be that the Straw Hat Crew travels back in time to the era of the Pirate King Gol D. Roger. They could meet him and learn some of his secrets, or even witness some of his greatest battles. Another plot could involve traveling to a time when the world was in a different state, like before the Grand Line was discovered, and they have to adapt to a whole new set of challenges and rules. Maybe they accidentally change something in the past that has a huge impact on the present and they have to figure out how to fix it.
The Straw Hat crew would be in for a big shock at first. But then they would quickly rally around Luffy and Nami. Robin might offer her wisdom on raising a child and share stories from her own past. Franky would build amazing things for the baby like a super - safe crib or a high - tech stroller. Brook would play music to soothe the baby. And they would all work together to make sure the baby has a great life on the ship.